1. Field of the Invention
The invention relates to a vehicle braking force control device. More specifically, the invention relates to a vehicle braking force control device which executes a brake assist control that generates a braking force higher than that at a normal time, when an operation state of a driver with respect to a brake operating member is a predetermined state.
2. Description of the Related Art
One braking force control device for a vehicle such as an automobile is, for example, the braking force control device disclosed in Japanese Patent Application Laid-open 9-290743, which is also an application of the applicant of the present invention. In particular, this braking force control device is configured such that a brake assist control is executed when a master cylinder pressure is equal to or more than a reference value thereof, and a rate of change of the master cylinder pressure is equal to or more than a reference value thereof. Moreover, another braking force control device is configured such that when a master cylinder pressure is equal to or more than a reference value thereof, and after a rate of change of the master cylinder pressure becomes equal to or more than a reference value thereof, a brake assist control is started when the rate of change of the master cylinder pressure becomes equal to or less than a pressure rise gradient of an oil pump.
According to the braking force control devices described above, the brake assist control is executed when a rapid braking operation is performed by a driver when, for example, avoiding a potential risk. A brake pressure of each wheel is controlled to a higher pressure than the master cylinder pressure, and thus, delayed braking of the vehicle caused by a lack of depression force of the driver can be effectively prevented.
Generally speaking, there are a variety of brake operations performed by a driver when braking a vehicle. Once example is so-called tapping, where a depression amount is decreased relatively rapidly after the brake pedal is depressed relatively rapidly. When this tapping brake operation is performed, there is no need for the brake assist control to be executed.
However, in the aforementioned conventional braking force control devices, fundamentally, when the master cylinder pressure is equal to or more than the reference value thereof and the rate of change of the master cylinder is equal to or more than the reference value thereof, brake assist control is executed. Therefore, as a result of the tapping brake operation being performed by the driver, a problem occurs where the brake assist control is executed unnecessarily when the master cylinder pressure is equal to or more than the reference value thereof and the rate of change of the master cylinder pressure becomes equal to or more than the reference value thereof.
Furthermore, a condition where a negative pressure of a vacuum type booster of the brake device is decreased to a lower value than a normal value will be described, as an example. In this case, even when the driver performs a rapid braking operation, the master cylinder pressure and the rate of change thereof do not become as high as when the negative pressure of the booster is the normal value. As a result, there are cases where the brake assist control is not executed despite a condition existing where it is preferable that brake assist control be executed.